1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a high density magnetic recording medium.
2. Description of the Prior Art
In recent years, there has been an increasing demand for high density recording media. In place of conventional magnetic recording media coated with magnetic paints, thin magnetic metal layers formed by plating, vapour deposition or sputtering without using a binder, are expected to be prospective recording media. The limit of the magnetic recording density is greately affected by the proportion of the demagnetizing field to the coercive force of the thin layer, and the strength of the demagnetizing field is related to the saturated magnetic flux density and the thickness of the thin layer. Further, it is necessary that the SN ratio is at least at an allowable level. As compared with the conventional coating type magnetic recording media, the thin magnetic metal layers having a greater saturated magnetic flux density, whereby a higher output is obtainable and a greater SN ratio is obtainable. Further, the thickness of the layer is thin and a relatively high coercive force is obtainable, whereby it is possible to substantially reduce the demagnetization of the magnetic recording.
As the major material for the thin magnetic metal layers, Co alloys such as Co--Si, Co--Fe--Ni and Co--Cr are commonly used. With these materials, however, there are some problems with respect to their costs and uncertainty of the steady supply of the raw materials in the future. These problems constitute a stumbling block against mass production.
Reflecting the recent trend for energy and resource conservations, various researches on the non-Co magnetic materials in which no Co is used, have been made. For instance, in the field of permanent magnets, it has been found that a magnet made of a Mn--Al--C alloy provides superior characteristics, as disclosed in Japanese Examined Patent Publication No. 31448/79.
Likewise in the field of thin layers, it has been reported that thin Mn--Al and Mn--Al--Cu layers formed by vapour deposition provide a maximum saturated magnetic flux density of 1,000 G and a maximum coercive force of 1,250 Oe and thus have excellent properties as magnetic recording media (for instance, in Journal of Applied Magnetic Science Association, Vol. 5, No. 2, 1981).
However, the Mn--Al and Mn--Al--Cu thin layers formed by a vapour deposition method have the following drawbacks. Namely, in the vapour deposition method, it is necessary to maintain the substrate at a temperature as high as from 300.degree. to 400.degree. C. at the time of vapour deposition. Therefore, a polymer film can not thereby be used as the substrate, and the method can not be applied to a magnetic tape or a flexible disc. Another substantial disadvantage is that the saturated magnetic flux density thereby obtainable is lower by about an order of tens than that obtainable by a Co thin film.